Minor UI tweaks relating to Channels

Updates

• Revised TabBar styling (spacing and increased box shadow visibility)
• Improved channel header layout (font sizes, alignment, colour, spacing and so on)
• Empty channel newsfeed - display an improved placeholder where newsfeed is empty
• Empty membership tier list - display placeholder
• Channel about screen: only show labels (e.g. Bio, Badges, Social Media links etc) where there is an associated value
• Improve input field styling within channel settings / about info & bio

To-dos

See #3144 (closed)

• Make channel bio an observable, so when a bio is updated via channel settings, it's reflected immediately within with the about tab
• Make feed list an observable within a channel feed. After a new post is created from within a user's channel, it should appear immediately within the feed. i.e. the user should not need to pull to refresh to see their new post